1. Field of the Invention
The present invention relates to a fail-safe system and is concerned particularly, although not exclusively, with a fail-safe system for use in measurement and control apparatus incorporating level detection apparatus.
2. Discussion of Prior Art
Fail-safe systems are used in measurement and control apparatus such as is used, for example, in a steam raising plant, where the level of water in a steam generation vessel is of critical importance. Level detection apparatus comprising a number of sensors immersed in water or steam is employed to monitor the water level. An example such level detection apparatus is described in our United Kingdom Patent No 1056032 in the name of Central Electricity Generating Board. Known fail-safe systems typically aim to output a fail-safe condition if there occurs a failure in the sensing circuit or else, for example, a loss of power. Such fail-safe systems are often extended so that a fail-safe output also results in the event of failure of a key specified component.
A drawback with the known fail-safe systems is that the failure of a component other than that which is specified will not be detected and will not automatically lead to a fail-safe output from the system.